Disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and servo sectors. The servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo control system to control the velocity of the actuator arm as it seeks from track to track.
Because the disk is rotated at a constant angular velocity, the user data rate is typically increased toward the outer diameter tracks (where the surface of the disk is spinning faster) in order to achieve a more constant linear bit density across the radius of the disk. To simplify design considerations, the data tracks are typically banded together into a number of physical zones, wherein the user data rate is constant across a zone, and increased from the inner diameter zones to the outer diameter zones. This is illustrated in FIG. 1A, which shows a prior art disk format 2 comprising a number of data tracks 4, wherein the data tracks are banded together in this example to form three physical zones from the inner diameter of the disk (Z0) to the outer diameter of the disk (Z2).
The prior art disk format of FIG. 1A also comprises a number of servo sectors 60-6N recorded around the circumference of each data track. Each servo sector 6, may comprise a preamble 8 for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark 10 for storing a special pattern used to symbol synchronize to a servo data field 12. The servo data field 12 stores coarse head positioning information, such as a track address, used to position the head over a target data track during a seek operation. Each servo sector 6i may further comprise groups of servo bursts 14 (e.g., A, B, C and D bursts), which comprise a number of consecutive transitions recorded at precise intervals and offsets with respect to a data track centerline. The groups of servo bursts 14 provide fine head position information used for centerline tracking while accessing a data track during write/read operations.
In the embodiment of FIG. 1A, the data rate of the servo sectors 60-6N changes in each physical zone similar to the data sectors in order to improve format efficiency. However, a zoned servo sector format may be problematic when generating the servo timing windows for detecting the servo sectors (e.g., the servo gate window and the sync mark window as shown in FIG. 1B). If the servo timing windows are generated using the synchronous clock of the servo channel (the disk locked clock), the servo frequency synthesizer may create a transient in the servo timing windows during a zone-crossing seek operation. That is, when the head crosses a servo zone boundary and the servo frequency synthesizer is reprogrammed to operate at the new zone frequency, there may be a transient (a time shift) in the timing windows resulting in loss of servo synchronization.